The present invention relates in general to semiconductor devices and, more particularly, to high frequency and analog integrated circuits made with bipolar transistors.
Many analog integrated circuits are fabricated with bipolar transistors because of their characteristic low noise operation as well as high gain and frequency response. For example, receiver circuits in wireless communications devices typically operate at frequencies of 2.4 gigahertz or more and require a high voltage gain and low noise. In many cases, integrated circuits having bipolar transistors are the preferred choice for these applications.
Most bipolar integrated circuits are optimized to produce high performance vertical NPN transistors. The complementary PNP transistors are available, but typically only as lateral devices whose base width is limited to the minimum feature size of the process. Because they have long base widths, such lateral PNP transistors operate with a low frequency response. Some integrated circuits provide vertical PNP transistors, but these devices have collectors that are formed in a grounded substrate, and therefore are usable only as emitter followers. Vertical PNP transistors having independent, unconnected collectors usually require numerous costly processing steps that result in too high a fabrication cost for many applications.
Hence, there is a need for an integrated circuit that provides both high performance NPN and PNP transistors and which can be manufactured at a low cost.